The operational characteristics of a laser source such as a semiconductor diode laser can be stabilized by reflecting a portion of the light emitted by the laser source back into the laser source's internal cavity. In applications where the emitted light of a laser source is coupled into an optical waveguide such as an optical fiber, these reflections can be obtained by forming reflectors in the optical waveguide. One common reflector is known as a fiber Bragg grating.
The operational characteristics of the laser source are said to be stabilized in the sense that the wavelength of the emitted light can be kept within a desired bandwidth and the emitted light power of the laser source is seen to vary more smoothly with variations laser drive current, operating temperature, component aging, etc. If the laser source is used to pump a gain medium such as a fiber amplifier or a fiber laser, for example, the stabilized operational characteristics manifest themselves as reduced noise in the pumped gain medium.
The operational characteristics may be stabilized by reflecting light back into the internal cavity of the laser source in such a manner that the laser source is forced to operate in a mode known as coherence collapse. Additional information regarding the operation of a laser source in coherence collapse is provided in U.S. Pat. No. 5,563,732, for example, which is incorporated herein by reference.
One requirement for coherence collapse is that the amplitude of the reflected light must be large enough to perturb the longitudinal mode hopping behavior of the laser source. An adequate amplitude can be achieved by using a reflector with a sufficiently high reflectivity level and, in many applications, a reflectivity level from about 2% to about 5% is sufficient. For example, see U.S. Pat. Nos. 5,485,481 and 5,724,377, both of which are incorporated herein by reference. Preferably, the level of reflectivity is set no higher than that necessary to maintain coherence collapse because the reflected light represents power lost from the application for which the laser source is used.
Another requirement for coherence collapse is that the reflected light must not be fully coherent in phase with the light emitted from the laser source. Phase incoherence between emitted light and reflected light can be achieved by ensuring the reflected light is returned by a reflector that is formed at an "optical distance" from the laser source that is approximately equal to or greater than the so called "coherence length" of the laser source. The coherence length depends upon a number of factors including the center frequency and bandwidth of the reflector, the reflectivity levels of the reflector and front facet of the laser source, and the magnitude of the drive current used to operate the laser source. In many applications, the coherence length of a semiconductor diode laser source is 50 cm. or more. The term "optical distance" refers to the distance that is measured along the optical path of the light.
Unfortunately, some systems that use reflectors to stabilize a laser source sometimes fail in the sense that the laser source drops out of coherence collapse and operates for an appreciable length of time in one of a number of longitudinal modes with occasional jumps between modes. Some applications cannot tolerate the noise that results from such operation; therefore, there is a need for an efficient low-cost way to stabilize a laser source that avoids these failures.
It has been determined that the cause of these failures is related to the polarization characteristics of the laser source and the optical waveguide. Three characteristics are particularly important: (1) laser sources like semiconductor diode lasers emit light that is polarized; (2) if this light is coupled into an optical waveguides such as birefringent optical fiber that is insensitive to polarization, the polarization of the emitted light is allowed to rotate as it travels along the waveguide and the polarization of the reflected light is also allowed to rotate as it travels along the waveguide from the reflector to the laser source; and (3) laser sources like semiconductor diode lasers are insensitive or "blind" to reflected light that is polarized orthogonally to the polarization of the light emitted by the laser source.
A semiconductor diode laser, for example, is more easily forced into coherence collapse if the reflected light is polarized in the plane of the diode junction. Conversely, coherence collapse is much more difficult if not impossible to achieve if the reflected light is polarized orthogonally to the plane of the diode junction. Under certain conditions, a polarization-insensitive optical waveguide will allow the polarization of the light reflected back to the laser source to be oriented substantially orthogonal to the polarization of the emitted light. If the laser source is insensitive to reflected light with such a polarization, the laser source will operate in a free-running state with erratic jumps in wavelength and operating modes.
The amount by which the polarization of reflected light rotates depends on a number of factors including transmission characteristics of the optical waveguide, length of the optical waveguide between the laser source and the reflector, wavelength of the light emitted by the laser source, and center frequency and bandwidth of the reflector. Generally speaking, it should be possible to arrange the optical waveguide and the reflectors to cause reflected light to be polarized orthogonally to the polarization of the emitted light. For example, additional polarization rotation can be obtained by coiling or otherwise stressing the optical fiber to induce birefringence into the waveguide.
One solution for this problem is to use a polarization-sensitive optical waveguide that maintains polarization so that reflected light and the emitted light have the same polarization orientation. For example, see U.S. Pat. No. 5,659,559, which is incorporated herein by reference. The use of polarization-maintaining (PM) waveguides is generally not attractive, however, because the cost of PM waveguides is higher and the alignment of PM waveguides is more difficult than that encountered with non-PM waveguides.
Another solution for this problem is to form the reflector in the waveguide at an optical distance from the laser source that is so small that the polarization of the light is not able to rotate by as much as ninety degrees in a round trip. If an optical waveguide such as optical fiber is tightly coiled, however, enough birefringence can be induced into the fiber to reduce this optical distance to perhaps 10 cm. or less. Unfortunately, this solution conflicts with the need to separate the reflector from the laser source by an optical distance that exceeds the coherence length of the laser source so that coherence collapse can be achieved.
Yet another solution, or at least a partial solution, is to increase the amplitude of the reflected light so that, as the polarization of the reflected light approaches orthogonality, the non-orthogonal component of the light has sufficient amplitude to stabilize the laser source. It is believed that the required increase varies inversely with the cosine of the polarization angle. For example, if a unit amplitude of light that is optimally polarized (polarization angle of zero degrees) is just sufficient to stabilize a laser source, then two units of amplitude would be required for a polarization angle of sixty degrees and about six units of amplitude would be required for a polarization angle of eighty degrees. Unfortunately, higher levels of reflectivity reduce the effective output power of a laser source. In addition, this solution does not stabilize operation as the polarization angle approaches ninety degrees.